The present invention pertains to lithography, and in particular pertains to a system for and method of rapidly and cost-effectively performing lithographic exposures in the manufacture of devices.
The process of manufacturing certain micro-devices such as semiconductor integrated circuits (ICs), liquid crystal displays, micro-electro-mechanical devices (MEMs), digital mirror devices (DMDs), silicon-strip detectors and the like involves the use of high-resolution lithography systems. In such systems, a patterned mask (i.e., a reticle) is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) that passes through an illumination system that achieves a high-degree of illumination uniformity over the illuminated portion of the mask. The portion of the radiation passing through the mask is collected by a projection lens, which has an image field (also referred to as a xe2x80x9clens fieldxe2x80x9d) of a given size. The projection lens images the mask pattern onto an image-bearing workpiece. The workpiece resides on a workpiece stage that moves the workpiece relative to the projection lens, so that the mask pattern is repeatedly formed on the workpiece over multiple xe2x80x9cexposure fields.xe2x80x9d
Lithography systems include an alignment system that precisely aligns the workpiece with respect to the projected image of the mask, thereby allowing the mask to be exposed over a select region of the workpiece. In most cases, the mask image needs to be precisely aligned to a pre-existing exposure field on the workpiece to provide the juxtaposed registration necessary to build up layers of the device being fabricated.
Presently, two types of lithography systems are used in manufacturing: step-and-repeat systems, or xe2x80x9csteppers,xe2x80x9d and step-and-scan systems, or xe2x80x9cscanners.xe2x80x9d With steppers, each exposure field on the workpiece is exposed in a single static exposure. With scanners, the workpiece is exposed by synchronously scanning the work piece and the mask across the lens image field. An exemplary scanning lithography system and method is described in U.S. Pat. No. 5,281,996. The projection lenses associated with steppers and scanners typically operate at 1xc3x97 (i.e., unit magnification), or reduction magnifications of 4xc3x97 or 5xc3x97 (i.e., magnifications of xc2x1xc2xc and xc2x1⅕, as is more commonly expressed in optics terminology).
The ability of a lithography system to resolve (or, more accurately, xe2x80x9cprintxe2x80x9d) features of a given size is a function of the exposure wavelength: the shorter the wavelength, the smaller the feature that can be printed or imaged. To keep pace with the continuously shrinking minimum feature size for many micro-devices (particularly for ICs), the exposure wavelength has been made shorter. Also, historically the device size has increased as well, so that the lens field size has steadily grown. The resolution of the lithography system also increases with the numerical aperture (NA) of the projection lens. Thus, in combination with reducing the exposure wavelength, the numerical apertures of projection lenses tend to be as large as can be practically designed, with the constraint that the depth of focus, which decreases as the square of the NA, be within practical limits.
Until fairly recently, semiconductor industry roadmaps predicted that lithography system field sizes would continue to increase to accommodate the increasing overall device size of memory and micro-processor chips. This trend has significant cost implications for manufacturing. In the case of smaller circuits, multiple devices would be fitted into a single exposure field. Although devices have generally grown in size, they have not grown as fast as anticipated by the industry roadmap makers, and the size of the minimum features used in the devices has shrunk faster than originally expected. The field sizes of the current generation of step-and-scan systems is more than adequate for the next few generations of memory, but the rapidly shrinking minimum geometry sizes are making it very difficult to obtain masks.
In order to provide some relief to the mask makers, the latest International Technology Roadmap update shows a minimum lens image field size of 25 mmxc3x9732 mm through 2003 changing to 22 mmxc3x9726 mm for 2004 through 2013. This decrease in the required field size allows the reduction magnification to be increased from 4xc3x97 to 5xc3x97, thus providing relief to the mask maker.
State-of-the-art lithography systems constitute some of the most complex machinery ever built, and as a consequence, are extremely expensive. Also, a good deal of effort and expense is also required to maintain and service lithography systems in the manufacturing environment. While it is relatively easy to build various kinds of experimental lithography systems in the laboratory for research and development purposes, it is an immense challenge to develop a lithography system for manufacturing purposes that is affordable and that operates in a cost-effective manner as determined by xe2x80x9ccost-of-ownershipxe2x80x9d considerations.
Choices between different types of lithography equipment are generally made on the basis of their relative cost-of-ownership. This cost-based model takes into account the cost of procuring, operating and maintaining a given lithography system in a manufacturing environment. The cost-of ownership is determined by considering various factors that relate directly to the properties of the lithography system and how the system is used in the manufacturing environment. There are a number of sophisticated cost-of-ownership models, an example being the SEMATECH Lithography Cost of Ownership Model, which take into account scores of different factors in performing the cost-of-ownership calculation. However, reliable cost-of-ownership trends can be obtained by examining a few key factors, such as stepper cost, mask cost, field size, system throughput (defined below), and the number of workpieces processed for a given mask.
It is known that the cost of the projection lens and illuminator for a lithographic system increases roughly as the cube of the lens field size. The cost of a state-of-the-art stepper having a lens capable of 0.13 micron resolution and operable at a 193 nm exposure wavelength with a field size of 22 mmxc3x9722 mm is roughly divided evenly between the lens and the rest of the system. The latter includes, the mask and wafer handling systems, the mask and workpiece stages, the laser exposure source, alignment systems, etc. Integrating all of these components into a usable system with installation and warranty leads to system prices of between about $10 M and $20 M.
Lithography system xe2x80x9cthroughputxe2x80x9d (i.e., the number of workpieces capable of being processed per hour) is one of the most significant factors in the cost-of-ownership calculation. To first order, the throughput of a conventional lithography system increases as the square of the exposure field size (diameter). Historically, throughput has been limited in part by the brightness of the radiation source, which must deliver a sufficiently high and uniform dose of radiation to the wafer for every exposure field. However present-day radiation sources are typically narrow-band, pulsed excimer lasers operating at 2000-4000 Hz and are far brighter than the arc sources used previously. The pulse-to-pulse uniformity of a typical excimer laser suitable for use in lithography is quite poor, i.e., 8-10% (3"sgr").
As lithography systems typically require illumination uniformity over the mask of less than 1% (3"sgr"), numerous (e.g., 100) pulses from the radiation source are averaged together to achieve the required uniformity. With laser repetition rates of several thousand hertz, the exposure times of step-and-scan systems have been reduced considerably so that the throughput rate of current lithography systems is primarily limited by stage motion and settling times (for steppers) and acceleration and scanning times (for step-and-scan systems). Reticle stage turn-around time, including settling and overscan, takes about 140 ms for each field for a modern step-and-scan system. This is similar to a stepper""s wafer stage move-and-settle time. In a step-and-scan system, the most difficult kinetic problems are usually encountered in the mask stage, which must accelerate and move X times faster than the wafer in proportion to the magnification reduction ratio X.
Another significant cost associated with a lithography system is that of the mask. The mask cost is roughly proportional to the square of the lens field size (diameter). This determines the amount of information embedded in the mask and therefore the write times, the inspection times and the probable number of defects. In certain applications, such as high-volume DRAM or microprocessor manufacture, the cost of the mask is a minor expense compared to the cost of depreciating the lithography system. However, for specialized (xe2x80x9cfoundryxe2x80x9d) applications, (e.g., the making of specialty devices such as digital signal processors, customized control circuits, etc.), the mask cost dominates the cost of lithography. A set 30 of masks for a foundry application may cost $1,000,000 or more, including the cost of the blanks, pellicles, set-up, writing, inspection and repair.
The numbers of workpieces per mask (WPM) to be run on a given lithography system is another key cost-of-ownership factor. The WPM can vary from 3 to 3,000 in a foundry application. The point where mask costs and lithography tool depreciation cost become equal depends on a variety of factors. As a general rule, for a lithography system having a large field size (e.g., 22 mmxc3x9722 mm) and running 3000 WPM or less, the cost-of-ownership is dominated by the mask costs.
The industry trend and conventional wisdom has been to design lithography systems to optimally serve the DRAM (memory) and microprocessor segments of the industry. With either step-and-repeat or step-and-scan technologies, these large segments of the industry are well served, and achieve the lowest cost-of-ownership with the field sizes recommended in the International Technology Roadmap. Other applications, which might be better served with systems having more modest field sizes, have generally been ignored by semiconductor equipment manufactures.
It would be advantageous from a cost-of-ownership viewpoint, for example, to be able to reduce the size of the lens field to some optimum value, taking into account the fact that mask costs dominate many applications. However, stepper throughput is roughly proportional to the square of the field size, which tends to offset somewhat, but not entirely, the savings in mask cost that are achieved with smaller field sizes. Also there are always some chips that are very large that could not be contained in the field of a small-field system. To date the added development cost, and the reduced potential market size for a small-field lithography systems has held back the development of such a system despite its cost of ownership advantages. Also the cost-of-ownership advantages of small-field systems are not well known.
Accordingly, what is needed is a lithography system capable of rapidly and cost-effectively exposing wafers in cases where there is less than 3000 workpieces per mask (WPM). A further improvement would be to achieve a high throughput, even where the system utilizes a projection lens with a relatively small lens field.
The present invention pertains to lithography, and in particular pertains to a system for and method of cost-effectively performing lithographic exposures in the manufacture of devices. The cost-effectiveness is further enhanced by a method of performing lithographic exposures very rapidly.
An aspect of the present invention involves the use of lithography lenses in lithography systems for manufacturing, wherein the lenses have lens (image) field sizes that are substantially smaller than that of conventional lithography systems and which, therefore, allow for the use of masks that contain substantially less information than masks used with conventional systems. Use of a small-field lithographic lens saves a considerable amount of money in the initial price of the lithography system for manufacturing. The savings on the smaller mask depend on the number of substrates to be exposed using the mask, which are appreciable on jobs involving fewer than 3000 substrates per reticle set. Although the smaller lens field size also leads to a lower throughput, the savings in the cost of the lens and on masks more than offsets this disadvantage.
Another aspect of the present invention involves using a novel xe2x80x9ccontinuous lithographyxe2x80x9d exposure mode. With this mode, it is possible to achieve exposures using single pulses of radiation and eliminate the throughput disadvantage associated with small lens fields. Use of a smaller-than-conventional lens field size facilitates single pulse exposures because the total amount of energy required per pulse is proportional to the exposure area. Thus, a smaller lens field size requires a smaller and therefore less expensive laser or other pulsed radiation source such as a flash lamp. Also the useful life of the lens may be limited by the high energy contained in each radiation pulse. High-energy pulses of deep UV radiation tend to compact glass over an extended period of time and eventually lead to unacceptable wavefront errors in the lens. A smaller lens field size opens up the number of design possibilities to include designs having fewer refractive lens components and more reflective lens elements. Since it is the refractive lens elements that have limited life, a design having fewer refractive elements can generally be expected to have a longer useful life. This makes for a lithography system that allows for the cost-effective use of smaller-than-usual lens (image) fields, which translates into less expensive projection lenses, and more generally, less expensive manufacturing of devices, such as semiconductor integrated circuits and the like.
Accordingly, an aspect of the invention is a lithography system for cost-effectively exposing a workpiece with successive images of a mask to be used to process 3000 substrates or less, and to form a plurality of exposure fields using a burst of radiation pulses to expose each field. The system includes, in order along an optical axis, a radiation source for providing radiation pulses. An illuminator is arranged to receive the pulses of radiation and substantially spread each pulse of radiation uniformly over the mask plane with a spatial uniformity of xc2x110% or less. Also included is a mask holder capable of supporting a mask to be substantially uniformly illuminated by each burst of radiation pulses exiting the illuminator. The system also includes a projection lens having an object plane arranged at or near the mask, an image plane arranged at or near the workpiece, and an image field within the image plane. The projection lens image field is sized to cover an image area less than half that of conventional lithography systems, so that the mask requires less than half as much detail as a mask used in a conventional lithography system. The projection lens is arranged to receive radiation transmitted by the mask to form a mask image on the workpiece within the image field. A workpiece stage is provided for supporting the workpiece at or near the image plane. The workpiece stage is adapted to step the workpiece so as to allow for a burst of radiation pulses to expose corresponding adjacent exposure fields with the successive mask images.
Another aspect of the invention is a cost-effective lithography system for rapidly exposing a workpiece with successive images of a mask to form a plurality of exposure fields using a single pulse of radiation per exposure field. The system includes, in order along an optical axis, a radiation source for providing pulses of radiation having a pulse-to-pulse uniformity of xc2x110% or less. An illuminator is arranged to receive the pulses of radiation and spread each pulse of radiation substantially uniformly over the mask plane. Also included is a mask holder capable of supporting a mask to be substantially uniformly illuminated by each pulse of radiation exiting the illuminator. The system also includes a projection lens having an object plane arranged at or near the mask, an image plane arranged at or near the workpiece, and an image field within the image plane. The projection lens is arranged to receive radiation transmitted by the mask to form a mask image on the workpiece within the image field. A workpiece stage is provided for supporting the workpiece at or near the image plane. The workpiece stage is adapted to move the workpiece over a scan path at a velocity that allows for single pulses of radiation to expose corresponding adjacent exposure fields with the successive mask images without appreciably smearing the mask images.
A further aspect of the invention is a pulse stabilization system, which can be arranged downstream of the radiation source to stabilize the pulse-to-pulse uniformity of the radiation pulses from the radiation source.
In another aspect of the invention, the cost-effective lithography system of the present invention includes a radiation source controller operatively connected to the radiation source, and a workpiece stage position controller for controlling the movement of the workpiece stage. The operation of the radiation source and the movement of the workpiece stage are facilitated by a metrology device that measures the precise location of the workpiece stage relative to a reference. The metrology device is electrically connected to the radiation source controller and provides the position information necessary to coordinate the pulses of radiation with the movement of the workpiece over the scan path.
In another aspect of the invention, the workpiece includes an image-bearing surface, and each pulse of radiation has energy sufficient to expose but not ablate the image-bearing surface.
A further aspect of the invention is a method of forming multiple exposure fields on a workpiece with a projection lens. The projection lens has an object plane at which a mask having a pattern is supported, and an image plane with an image field within which a mask image is formed. The method comprises the steps of aligning the workpiece relative to the image field, then irradiating the mask with a plurality of radiation pulses. The radiation pulses preferably have good pulse-to-pulse uniformity (i.e., little variation in exposure dose), i.e., 10% (3"sgr") or less, and more preferably, 1% (3"sgr") or less. The pulses also preferably have good spatial uniformity of illumination over the object plane of 10% (3"sgr") or less, and more preferably 1% (3"sgr") or less. The method further includes the step of collecting, with the projection lens, the portion of the pulses of radiation that are transmitted by the mask so as to form a separate mask image with each pulse of radiation. During the collecting step, the workpiece is continuously moved over a scan path in the image plane so that each mask image forms a corresponding separate exposure field.
Another aspect of the invention is the above-described method, which further includes the step of aligning the mask image to one or more pre-existing separate exposure fields on the workpiece.
Another aspect of the invention is the above-described method, including the step of sending the radiation pulses through a pulse stabilization system to improve the pulse-to-pulse uniformity.
A further aspect of the invention is a method of rapidly forming a plurality of sequentially arranged exposure fields on a workpiece with a projection lens having an object plane, an image plane and an image field. The method includes the steps of supporting a mask having a pattern at or near the object plane, and arranging a workpiece stage for the workpiece on which the sequentially arranged exposure fields are to be formed, to be movable within the image plane over a scan path relative to the image field. The method also includes irradiating the mask with pulses of radiation and collecting the transmitted radiation with the projection lens to form a mask image within the image field for each radiation pulse. The workpiece stage is moved continuously over the scan path during the mask irradiation so that adjacent radiation pulses form adjacent exposure fields.
Another aspect of the invention is the above-described method, wherein adjacent exposure fields are formed in registered juxtaposition with pre-existing exposure fields formed on the workpiece.